Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

Chapter Nine
Design of Permanent Joints (Welding) )‫تصميم الوصالت الدائمية ( اللحام‬
9–1 Welding Symbols
A weldment is fabricated by welding together a collection of metal shapes. The welds must be
precisely specified on working drawings, and this is done by using the welding symbol as shown in
Fig. 9–1,

Fig. 9–1

Figure 9–2 Arc- and gas-weld symbols.

Figure 9–3:Fillet welds. ‫( اللحام التراكبي‬a) The number indicates the leg size; the arrow should point only to one weld
when both sides are the same. (b) The symbol indicates that the welds are intermittent and staggered 60 mm along on
200-mm centers

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

Figure 9–4 indicates the around welding

Figure 9–7 A typical butt joint ‫اللحام التناكبي‬.

9–2 Butt and Fillet Welds

1- Butt Welds
Figure 9–7a shows a single V-groove weld loaded by the tensile force F. For either tension or
compression loading, the average normal stress is

(9-1)

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
From figure 9–7b, the average shearing stress is

(9-2)

where h is the weld throat and l is the length of the weld, as shown in the figure. Note that the
value of h does not include the reinforcement

2-Fillet Welds
Consider the external loading to be carried by shear forces on the throat area of the weld, Figure
9–11. By ignoring the normal stress on the throat, the shearing stresses are inflated sufficiently to
render the model conservative.
For this model, the basis for weld analysis or design employs the shearing stress is:

Figure 9–11: Parallel fillet welds.

(9-3)

t : the welding line throat

h : the welding line height
l : the welding lind length

Example 9-1:
The figure shows a horizontal steel bar of thickness h
loaded in steady tension and welded to a vertical support.
Find the load F that will cause an allowable shear
stress, , in the throats of the weld.

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
Solution
Given, b = 50 mm, d = 30 mm, h = 5 mm, .
( ) ( ) ( )( )( )

9–3 Stresses in Welded Joints in Torsion

Figure 9–12 illustrates a cantilever of length l welded to a column by two fillet welds. The reaction
at the support of a cantilever always consists of a shear force V and a moment M.

Figure 9–12 Torsion in the welds

The shear force produces a primary shear in the welds of magnitude

(9-4)

where A is the throat area of all the welds.

The moment at the support produces secondary shear or torsion of the welds, and this stress
is given by the equation

(9-5)

Since the throat width of a fillet weld is 0.707h, the resulting second moment of area is then a
unit second polar moment of area, the relationship between J and the unit value is
(9-6)
can be taken from table 9-1

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
Table 9–1: Torsional Properties of Fillet Welds*

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Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

9–4 Stresses in Welded Joints in Bending

Figure 9–17a shows a cantilever welded to a support by fillet welds at top and bottom. A free-
body diagram of the beam would show a shear-force reaction V and a moment reaction M. The
shear force produces a primary shear in the welds of magnitude

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

( )

where A is the total throat area.

( )

( )

√ ( )

Figure 9–17: A rectangular cross-section cantilever welded to a support at

the top and bottom edges.

Table 9–2: Bending Properties of Fillet Welds*

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

Example 9 – 4 (Bending Welds) (P 9 - 48)

The figure shows a welded steel bracket loaded
by a static force F. Estimate the factor of safety,
if the allowable shear stress in the weld throat is
18 kpsi.

( ) ( ) ( )

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

( )( ) ( ( )) ( )

( )( )

Primary shear:

Secondary shear (the critical location is at the bottom of the bracket):

=5.173 kpsi

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.

9–5 The Strength of Welded Joints

The matching of the electrode properties with those of the parent metal is usually not so
important as speed, operator appeal, and the appearance of the completed joint.
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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
The properties of electrodes vary considerably, but Table 9–3 lists the minimum properties for some
electrode classes. It is preferable, in designing welded components, to select a steel that will result
in a fast, economical weld even though this may require a sacrifice of other qualities such as
machinability.
Under the proper conditions, all steels can be welded, but best results will be obtained if steels
having a UNS specification between G10140 and G10230 are chosen.
Example 9-2:
For the weldment shown in Fig., the electrode metal is
E7010, what is the allowable load on the weldment? .

Solution:
From table 9-6
( )

Table 9–3 Minimum Weld-Metal Properties

Table 9–4 Stresses Permitted by the AISC Code for Weld Metal

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
Table 9–5 Fatigue Stress-Concentration Factors, Kfs

IMPORTANT NOTES
 The best welded steels have a tensile strength in the hot-rolled condition in the range of 410 to
480 MPa, so a cold-drawn bar has its cold-drawn properties replaced with the hot-rolled
properties in the vicinity of the weld.
 Table 9–3 lists the minimum properties for some electrode classes.
 It is important to observe that the electrode material is often the strongest material present a
welded joint.
 Table 9–4 lists the formulas specified by the code for calculating these permissible stresses for
various loading conditions
 Finally, remembering that the weld metal is usually the strongest, do check the stresses in the
parent metals.
Table 9-6

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
Example 9-3 (Torsion welds) (P 9-17)
A steel bar of thickness h, to be used as a beam, is welded to a
vertical support by two fillet welds as shown in the figure.
(a) Find the safe bending force F if the allowable shear stress in
the welds is tallow.
(b) In part a, you found a simple expression for F in terms of the
allowable shear stress. Find the allowable load if the electrode is
E7010, the bar is hot-rolled 1020, and the support is hot-rolled
1015.

Solution
(a) b = d =50 mm, c = 150 mm, h = 5 mm, and
 Primary shear, from Table 9-1, Case 2
Note: b and d are interchanged between problem figure and table figure. Note, also, F in kN and
in MPa
( )
( )( )
 Secondary shear (Table 9-1, Case 2)

( ) ( ( ) ( ) )

( )( )

( )( )

( ) ( )
( ) ( )

(b): (weld strength)

For E7010 from Table 9-6

1020 HR bar:
1015 HR support

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
Table 9-3, E7010 Electrode:
The support controls the design.
Table 9-4: ( ) ( ) ( ) ( )

The allowable load, from Eq. (*) is

EXAMPLE 9–5

A 12 mm by 50 mm rectangular-cross-section 1015
bar carries a static load of 73 kN. It is welded to a
gusset plate with a 10 mm fillet weld 50 mm long
on both sides with an E70XX electrode as depicted
in the figure. Use the welding code method.
(a) Is the weld metal strength satisfactory?
(b) Is the attachment strength satisfactory?

Solution
(a) From Table 9–6, )‫(من الكتاب‬.
Allowable force per unit length for a 10 mm E70 electrode metal is 1025 N/mm of weldment; thus
F = 1025l = 1025(50x2) = 102.5 kN
Since 102.5 > 73 kN, weld metal strength is satisfactory.
(b) Check shear in attachment adjacent to the welds.
= 190 MPa (Table A–20)
The allowable attachment shear stress is
From Table 9–4

( )
{
( )

The shear stress on the base metal adjacent to the weld is

( )( )

Since , the attachment is satisfactory near the weld beads.

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The tensile stress in the shank of the attachment is

( )

The allowable tensile stress , from Table 9–4, is and, with welding code safety level
preserved,

( )

Since , the shank tensile stress is not satisfactory

To overcome this problem a new dimensions or new material may be selected. let

( )

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 Mechanical Design Ch9: Welded Joints 3rd year - Mechanical. Engineering.
9–29 The permissible shear stress for the weldment
illustrated is 140 MPa. Estimate the load, F, that
will cause this stress in the weldment throat.

9–34 The attachment shown in the

figure is made of 1018 HR steel 12 mm
thick. The static force is 100 kN. The
member is 75 mm wide. Specify the
weldment (give the pattern, electrode
number, type of weld, length of weld,
and leg size).

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